In a working space of a machine tool, the magnitude of loads (including force and moment) and loading positions of members of the machine tool vary as the position of the machining point varies, so that a static stiffness distribution of the machine tool changes. The change in static stiffness at different machining positions may be described by a stiffness distribution. The magnitude of the static stiffness and the static stiffness distribution influence the machining precision directly (especially the stiffness distribution influences the shape precision of the machining surface directly), and influence the vibration characteristics of the machine tool at the same time.
In a design stage, the static stiffness and the static stiffness distribution of a design scheme may be predicted using an analysis method, and then the design scheme is revised according to the predicted results so that the stiffness and the stiffness distribution are improved.
One of the purposes of the static stiffness detection test for the numerical control machine tool is to provide data for evaluating the static stiffness of the machine tool, and the another one is to check and verify the correctness of the predicting method for the static stiffness and static stiffness distribution and the scheme revising method, so as to provide experimental means for researching and improving the prediction and the scheme revising method.
Regarding multi-axle joint numerical machine tools, various numerical machine tools have different principles for forming machining surface and different movement functions of servo axles of their feeding system, as well as different joint relations and numbers of joint axles for maintaining a strict movement relationship among numerical axles; loads carried by various multi-axle joint numerical machine tools have different properties, and various multi-axle joint numerical machine tools have different load changing rules at various machining positions, and different ratios among loads in respective directions. Therefore, loads simulation apparatuses and their corresponding detecting methods are different from one another for various multi-axle joint numerical control machine tools. For example, a 5-axle processing center having three linear movements in Z axle, Y axle and X axle, and two rotation movements around C axle and A axle, may machine various kinds of complex surfaces by the joint movement of the Z axle, Y axle, X axle C axle and A axle. In the actual machining, forces in three directions are applied to a machining point, and a load from a worktable is transmitted through the workpiece, and the worktable will carry six cutting loads (three forces Fx, Fy and Fz and three moments Mx, My and Mz); meanwhile, the main axle will also carry six cutting loads.
In a machine tool static stiffness test, cutting loads are replaced by simulation loads. The processing center static stiffness detection apparatus and the detecting method available both in China and abroad can only detect the static stiffness at one determined position, and cannot detect the static stiffness distribution. The 5-axle processing center may machine various kinds of complex surfaces through a joint movement of the Z axle, Y axle, X axle C axle and A axle, the position of a point to be machined varies during machining, and accordingly the positions of the members on Z axle, Y axle, X axle C axle and A axle vary, so that both the static stiffness at the main axle side and that at the worktable side change as the position of a machined point on a workpiece varies. The change in stiffness influences the shape precision of a machining surface directly, and thus it is necessary to detect the static stiffness distribution of a processing center.
The processing center static stiffness detection apparatuses and detecting methods both in China and abroad cannot apply six simulation loads to the main axle. Therefore the cutting load cannot be fully simulated at the main axle side. The worktable and the main axle carry a load and an anti-load, respectively, so that the cutting load at the worktable side cannot be fully simulated, though six simulation loads can be applied at this side. In the machine tool static stiffness test, the cutting loads are replaced by the simulation loads, simulation forces in three directions are applied to a loading point, and it is desired that both the main axle side and the worktable side of the processing center can endure six simulation loads for fully simulating six simulation loads of the cutting loads.